Production of active metals



Patented July 1%, i934.

PRODUCTION or ACTIVE METALS I John W. Marden and Ernest A. Lederer, East Orange, N. J assignors to Westinghouse Lamp Company, a corporation of Pennsylvania No Drawing.

Serial No. 218,570. 1933 Application September 9, 1927,

Renewed September 13,

7 Claims. (01. 250-475) This invention relates to the introduction of chemically active metals into evacuated or gasfilled containers, such as electron discharge devices, and more particularly to the introduction of alkali metals into such devices.

In the manufacture of electric devices it is desirable to introduce therein, a small quantity of an active substance in a pure state, such as an alkali metal for the purpose of cleaning up residual gases in evacuated containers, purifying rare gases in gas-filled containers, activating the filament by increasing the electron emissivity thereof, lowering the starting and operating potential of discharge lamps, rectifiers, etc., creating a residual metallic vapor in the envelope and for various other purposes.

Extreme difficulty has hitherto been experienced in attempting to introduce these highly volatile materials, particularly the alkali metals,

' such as caesium, potassium, sodium, etc., into the envelopes of various gas tight containers,

since in their free state, these metals combine very readily with the elements of the atmosphere and therefore cannot be handled to any extent.

The alkali metals caesium, potassium, sodium, lithium, and rubidium, vaporize at relatively low temperatures and if they are introduced in such devices prior to evacuation thereof, it is diflicult to maintain. them in a non-volatile state while baking out the envelope during the exhaust operation. Moreover, these metals oxidize so rapidly upon exposure to the atmosphere that they I cannot be introduced directly into the envelope in metallic form. caesium, for instance, burns up with great rapidity in the air.

Heretofore, the alkali metals have usually been introduced into the envelope, after exhausting, by vaporizing the same from a side tube 'or 40 container. It has also been proposed to introduce such metals into evacuated containers in the form of a stable compound which is subsequently decomposed by means of a suitable reducing agent in order to liberate the alkali metal in the 4 container.

Diiiiculty has been experienced with the compounds heretofore employed due to the absorption of moisture thereby prior to their introduction into the evacuated device. The water vapor is liberated at the time the decomposition takes place and deleteriously effects the operation of the device in which the alkali metal is introduced. For instance, in electron discharge devices em ploying thoriated tungsten cathodes, the water vapor causes oxidation of the thorium content of the cathode, destroying or seriously impairing its emission. It has not been possible to heat these compounds, in baking the device suificiently to eliminate all the Water therefrom.

In the manufacture of high vacuum devices it 69 is the usual practice to bake the electrodes in hydrogen prior to mounting to remove all traces of oxygen therefrom and during evacuation, to bake out the envelope at relatively high tempera- 5 tures to drive out all water vapor and occluded gases from the glass. It will be readily appreciated that it is useless to take these extreme pres cautions to eliminate all moisture from the device and to again liberate water vapor upon the F decomposition of the alkali metal compounds.

It is one of the objects of the present invention to provide a practical method of introducing highly volatile and strongly reactive metals, such as the alkali metals, into gas tight containers in r a pure state, without the simultaneous introduction of water vapor or the complete oxidation thereof.

Another object is to provide a method of introducing chemically active metals into evacuated containers in a non-hygroscopic condition and free from deleterious substances.

Another object is to provide a method of introducing an electro-negative gas into an electron discharge device employing an alkali metal as the active electron emitting material, for the purpose of assisting in retaining the alkali metal on the cathode of the device.

Other objects and advantages will hereinafter appear.

In accordance with this invention the chemically active metal, which it is desired to employ in the evacuated device, is introduced into the device in the form of a stable moisture free or anhydrous compound which is subsequently reduced to liberate the metal inpure form and free from water vapor or other substance which might deleteriously effect the, operation or life of the device. The stable compound 'may be mixed in powderedform with finely divided metalloid, such as silicon or boron which serves-as a reducing agent. The mixture may be pressed into pills or other convenient form and be supported in a position in the device which is capable of being heated to a sufficient temperature to effect the reduction of the compound.

If desired, the compound and reducing agent may be enclosed in a capsule and the capsule welded or otherwise secured to a metal part of the device where it can be heated by high frequency induction currents.

The compounds whichwe prefer to employ are a dichromate or permanganate of the alkali metal, as for instance caesium dichromate or permanganate mixed with finely divided silicon in the proportion of one part of the alkali metal compound to three parts silicon.

The dichromate and the permanganate are stable and may be dried completely free from water vapor, and they do not reabsorb moisture upon standing. They are readily reduced by the metalloids, silicon and boron which may also be rendered gas and moisture free and which do not reabsorb any deleterious gases. Metallic reducing agents, such as magnesium, calcium, etc" have been employed heretofore, but such metals absorb gases such as hydrogen or carbon monoxide and in the case of calcium, water vapor from the air which are liberated in the envelope to the detriment of the device. Silicon and boron, .on the other hand, are very stable and may be heated at relatively high temperature, in air, to drive oif the gases and vapors without oxidation.

While commercial silicon or boron have proven satisfactory, we prefer to further treat the same to remove the gas contained therein, by boiling the finely powdered material in dilute hydrochloric acid until gas evolution entirely ceases. Various proportions may be used, but we prefer to employ about 50 grams of powdered silicon orboron in 100 cc. of a1. to 4% solution of hydrochloric acid. 7

After the gas has been completely eliminated, the metalloid is filtered out, washed and dried.

The silicon or boron may then be mixed in the above stated proportions with either a dichromate or permanganate of the metal it is desired to introduce. Upon the heating of this mixture, after it has been introduced into the envelope, the compound is reduced and the alkali metal liberated without the evolution'of any gases or impurities. except a small quantity of oxygen. The reaction is such, however, that the oxygen appears to be all liberated prior to the liberation'ofthe alkali metal, and may be removed by the pumps before the alkalimetal is set free in the envelope. In the case of the caesium permanganate and silicon mixture, the permanganatebreaks down upon initial heating, formingcaesium oxide and liberating free oxygen which may. bepumped from the device. -Upon continued heating, the caesium oxide isreduced by the silicon, liberating free metallic caesium. The reaction may be expressed as follows:

2CSMIlO4- CS20+2MQO2+3O 2osio+si+sio2+4cs. The temperature required to decompose the caesium compound is estimated at about 900 C;

We have found that this temperature may be greatly reduced by adding to the mixture a small proportion of a metallic reducing agent, insufiicient in quantity to give off a detectable amount of gas. We prefer to add to the mixture of the permanganate and silicon, about 1% of finely powdered aluminum, free from grease and as pure film of caesium metal thereon.

as is obtainable. The small addition of aluminum reduces the reaction temperature of the mixture to about 700 C. Commercial powdered aluminum should not be employed, since it is ordinarily ground under oil and contains large quantities of grease. We prefer to use filings made from pure aluminum rod.

In making caesium amplifier tubes at the present time, it is customary to thoroughly exhaust and bake out the tube and then to flush out the envelope with a small amount of oxygen introduced therein from an outside container. The

lament is flashed in the oxygen to oxidize the surface thereof to render it capable of retaining a The excess oxygen is then pumped out and the caesium introduced into the device. By employing caesium permanganate or dichromate or other oxygencontaining compound of the alkali metal, the step of introducing oxygen from a separate source may be eliminated and the oxygen supplied by the decomposition oi the compound employed to flush out the bulb. For this purpose, we prefer to use the permanganate.

It is understood that modifications may be made in the invention described herein without departing therefrom and We do not desire to be limited to the exact embodiments described.

That is claimed is:

1. The method of introducing alkali metals in electron discharge devices comprising introducing into the devices a mixture comprised of about one part alkali metal permanganate mixed with about three parts finely divided silicon, and a small proportion of aluminum, and heating the mixcure to cause a reduction of the permanganate with a liberation of the alkali metal.

2. The method introducing caesium in electron discharge devices comprising introducing into the devicesa mixture comprised of caesium permanganate finely divided silicon and aluminum, and heating the mixture to liberate the caesium by the reducing action of the silicon and aluminum on the caesium permanganate.

3. A getter for producing alkali metal deposits in evacuated devices comprising a mixture of a permanganate of the alkali metal mixed with finely divided silicon and aluminum.

7 4. A getter for producing caesium deposits in evacuated devices comprising a mixture of caesium permanganate, finely divided silicon and a small proportion of aluminum.

5. -An admixture for producing an alkali metal containing deposit in an electric discharge device comprising a non-hygroscopic compound of said alkali metal and a quantity of silicon.

6.. A mixture for producing a caesium containing deposit in an electric discharge device comprising a non-hygroscopic compound of caesium and a quantity of silicon.

7. A mixture for producing a caesium containing deposit in an electric discharge device comprising caesium dichromate and silicon.

JOHN W. MARDEN. ERNEST A. LEDERER.

III) 

